Support `ipv{n}.address=none` for `ovn` networks

[tomponline]

Support ipv{n}.address=none for ovn networks to allow completely isolated networks.

This would:

• Disable auto IP (DHCP/RA) for the relevant IP protocol(s).
• Disable DNS forwarding for the relevant IP protocol(s)
• Remove the relevant IP(s) from the router's external switch interface.
• Remove the relevant IP(s) from the router's internal switch interface.
• Disable router uplink connection entirely (if both IPv4 and IPv6 were disabled).